Casey Kelly’s Arizona Fall League season concludes

The line score was fine. In his final start of the Arizona Fall League season, Red Sox top prospect Casey Kelly allowed four runs (three earned) on five hits in five innings. He gave up one homer, walked one and struck out three.

But while that line might not exactly turn heads, Kelly finished the season flashing the sort of stuff that convinced the Sox that he had a very strong developmental year. His fastball topped out at 94 mph, and he showed a swing-and-miss curve and changeup. (Of Kelly’s 78 pitches, he elicited eight swings and misses, with five on curveballs and three on changeups.) As has been the case throughout this year, his mistakes were on fastballs up and over the plate, and against advanced hitters, that proved costly. All the same, it was a solid performance, giving Kelly three solid to strong outings (of his four starts) in Arizona.

Kelly’s AFL season is now over, and with it, so, too, is a 2010 campaign that was his first as a full-time pitcher. Between Double-A Portland and his time with the Peoria Javelinas of the AFL, Kelly logged 111 innings, going 4-5 with a 5.51 ERA, 92 strikeouts (7.5 per nine innings) and 39 walks (3.2 per nine innings).

While his 6.75 ERA in 16 innings in the AFL is gaudy, that was largely the byproduct of one terrible start (a two-inning, eight-run stinker) among his four outings. Otherwise, Kelly showed good stuff in Arizona and, perhaps more importantly, he was aggressive throwing strikes, walking just four in his 16 innings and allowing a pair of homers. (Both solid totals, considering the hitter-friendly environment of the AFL.) Considering that he was the second youngest pitcher in the fall league for top prospects (Kelly turned 21 on Oct. 4), he carried himself well, and he accomplished what he went to Arizona to do, tacking on 15-20 innings to his season to build his workload.

Now, with 2010 behind him, and the experience of a full season as a professional pitcher on his resume, Kelly is set up for a significant 2011 season. The Sox, with good reason, emphasized the fact that his stuff was better than his results in 2010.

Given that Kelly was dealing with youth (he was among the youngest pitchers in both the AFL and Double-A) and inexperience (he had 95 pro innings as a pitcher prior to this year), and that he was adjusting to physical development that took him from being a command pitcher to more of a power pitcher, it seems far to conclude that the numbers did not tell the whole story of his 2010 season.

But in 2011, with several of those adjustments having occurred, performance will likely be used as a meaningful barometer of his prospect status. If the physical gains of 2010 can translate into results, then his top-prospect status will be cemented further. If not, then there will be questions about whether he has been overhyped.

But, for now, such questions seem premature. This season was one of challenges, adversity and — the Sox believe — progress and development for a pitcher whose talent and makeup were both prominently on display throughout the year. For now, the numbers mean less in 2010 than the development that occurred for a young pitcher whose future, the Sox believe, remains extremely bright.